The present invention pertains to protection circuits for high voltage circuitry of television receivers or the like.
In television receivers, the relatively high operating voltages for the electron beam accelerating electrodes of the picture tube are conventionally developed by arrangements utilizing a transformer winding associated with circuitry for generating signals to control the deflection of the electron beams produced by the picture tube. Some of these high voltage generating arrangements may tend to develop excessively high voltages under certain conditions such as the failure of components within the arrangement itself or the development of abnormally high voltages on the AC power input lines. Such excessively high voltages may lead to component failures elsewhere in the receiver and, in some instances, to the emission from the picture tube of potentially harmful X-radiation. Therefore, protection circuits have been employed to disable the normal operation of the receiver such as by rendering the display produced by the receiver unviewable to provide a warning to a viewer when excessively high and therefore potentially harmful voltages are generated by the high voltage generating arrangement of the receiver. Unfortunately, many of these protection circuits may be removed or bypassed or themselves may undergo component failures without affecting normal receiver operation and therefore may not effectively provide a warning when excessively high voltages are being generated by the receiver.
In U.S. Pat. No. 3,906,304 entitled, "High Voltage Protection Circuit", issued in the name of the same inventor as the present inventor, there is described a protection circuit which is intended not to affect receiver operation under normal conditions but to provide a warning to a user by rendering the picture produced by the receiver unviewable under abnormal conditions. Moreover, this protection circuit is arranged to provide protection when it has been removed or bypassed or when it suffers a component failure itself. Specifically, the protection circuit comprises a rectifying device, e.g., a diode, and a switching device, e.g., an SCR (Silicon Controlled Rectifier) with an unconnected gate electrode, coupled together in anti-parallel relationship to form an ITR (Integrated Thyristor Rectifier). The ITR is desirably coupled between a winding of a transformer associated with the deflection circuit of the receiver and a filter capacitor of a power supply circuit for display processing portions of the receiver. Under normal conditions, the rectifying device is conductive only in one direction to charge the filter capacitor to a supply voltage sufficient to permit the development of a viewable display. However, under abnormal conditions, if the difference between the voltage developed across the filter capacitor and the voltage developed across the transformer exceeds a level dependent on the characteristics of the switching device, the switching device is rendered conductive, thereby inhibiting the development of sufficient supply voltage across the filter capacitor to permit the development of a viewable picture. Ideally, the characteristics of the switching device are selected so that it is rendered conductive at a level corresponding to the generation of excessively high and therefore potentially harmful deflection voltages. Unfortunately, the voltage at which the switching device in this arrangement is rendered conductive is not predictable and has a tendency to change with environmental conditions. Therefore, a receiver employing such an arrangement may be unnecessarily disabled from operating normally at an acceptable voltage or may be permitted to operate beyond the voltage at which harmful X-radiation is likely to be produced.
While numerous circuits are known (see, for example, U.S. Pat. No. 3,475,653 issued in the name of Odenberg et al.) which employ a reference voltage device coupled to a device having an SCR characteristic for controlling its conduction or firing point, none of these arrangements have structure which would enable them to be used in a power supply circuit to charge a filter capacitor to a supply voltage from a source of alternating voltage under normal conditions and to inhibit the development of the supply voltage under conditions of excessively high and potentially harmful alternating voltage with the additional feature of providing protection even when purposely tampered with.